The patron, whom the library declined to name, was surprised by the find. “Her concern was getting it back in the hands of the rightful owner,” Ms. Drogin said.

“It sure is a mystery,” Ms. Drogin said less than 24 hours after the discovery.

Library staff members don’t usually notice things left in books, unless they’re sticking out, like a bookmark. “We get so many books coming in every day, we just check them in and put them back on the shelves,” Ms. Drogin said.

The Times reached out to Mr. Buhner by phone in New Mexico, where he got a good laugh out of the find. “That was one of the most fun phone messages I’ve ever had about one of my books,” he told The Times. “It’s fascinating. That could only happen on Martha’s Vineyard.”

Mr. Buhner’s book was on display because ticks and Lyme disease are a big concern on the Island this time of year. The book offers herbal treatments for the tick-borne illness.

“Normally this is a book that’s downstairs at the back of the library,” Ms. Drogin said. “[Books about Lyme] circulate a lot, because a lot of people have it or know people who have it. This one had not been taken out since January.”

Speculation was rampant at the library after the money was turned in. “One of our co-workers thought maybe it was an anonymous donation to the library,” Ms. Drogin said.

Maybe it was all a test.

“I’m hoping that by turning it in, I get good karma,” Ms. Drogin said. Unsure of what to do with the money, Ms. Drogin and young adult librarian Laura Hearn turned it over to Laura Coit, the assistant library director.

“It’s not uncommon to find a bill, like a $1 or a $20 bill, used as a bookmark, and occasionally we have people who leave notes in books for fun as a random act of kindness,” Ms. Coit said. “If you were writing a short story, you might think someone wanted to give money to someone suffering from Lyme disease. One thing I’ve learned from working in a library: Truth is often stranger than fiction.”

It took a couple of days and some sleuthing to reunite the cash with its rightful owner. Ms. Coit said the library protects the privacy of its patrons, but in cases like this they do have ways to reach out to the last person who checked out the book.

On Friday, library officials did just that, finally cracking the case. “There’s really no story,” Beth Kramer, West Tisbury’s head librarian, said. “The money was returned to its rightful owner.”